Perhaps it would help, given recent events in U.S. politics, to review the rationale for U.S. security commitments to allies in Europe and the Far East.

The 20th century taught that our national security is inconsistent with the hegemony of a hostile power over either of those strategic regions. Preventing that might be expensive, but doing so collectively, through U.S.-led transpacific and transatlantic political-military structures, is affordable and far more cost-effective than isolationist or unilateral alternatives.

What's held these alliances together over the past seven decades or so has not been U.S. nuclear weapons or conventional might, or U.S. money, but U.S. political consensus - a shared belief across our society that the benefits of the U.S.-Japan defense treaty, the North Atlantic Treaty Organization, U.S. bases in Korea and other institutions outweigh the costs and risks.

That consensus represented a profound development for a country whose first president declared that "it is our true policy to steer clear of permanent alliance with any portion of the foreign world." But without it, the United States could neither have made long-term commitments nor had them taken seriously by both allies and enemies.

Today, the case for those alliances remains valid, given the aggressive postures of both Russia and China, and the threat of terrorism.

Yet the U.S. political consensus on which they ultimately rest is eroding - to the point where a major-party candidate for president finds it advantageous not to assuage public ambivalence about collective security, but to weaponize it, politically.

Republican Donald Trump's disparagement of U.S. alliances as one-sided "deals" that benefit wealthy free-riding allies, coupled with his equally blithe apologia for Russian strongman Vladimir Putin, is therefore no ordinary campaign tempest, but a historic event.

Perhaps something like this was bound to happen, even if leaders in the United States, Europe and Japan had done a perfect job managing collective security threats in recent years. Nothing lasts forever, political institutions least of all, and in the life of alliances, 67 years - NATO's age - is ancient.

Even at the height of the Cold War, 1976 Democratic presidential nominee Jimmy Carter pledged to pull U.S. troops out of South Korea, though he reneged as president. As that episode shows, foreign policy "retrenchment" is a recurring American temptation; it is especially so now, after years of war and economic underperformance. It certainly didn't help when President Barack Obama, generally a multilateralist, let slip his frustration with allied "free riders" to the Atlantic's Jeffrey Goldberg.

None of that reduces the radicalism, or the destabilizing potential, of Trump's attacks on long-standing U.S. security doctrine.

To the contrary, Trump's success might encourage more politicians to put U.S. alliances in play as a domestic issue, within both parties, whether he wins the White House or not.

A mere 49 percent of Americans have a favorable view of NATO, according to a 2015 Pew Research Center survey; only Spaniards were less enthusiastic among NATO- member publics. Majorities of both Democrats and Republicans support Trump's view that Germany should do more for the alliance, militarily. As for Asia, 64 percent of Americans favor keeping the U.S. military presence at current levels, according to a 2015 Chicago Council on Foreign Relations survey; yet fully 22 percent want to decrease it, despite threats from North Korea and China.

To be sure, the 2016 Democratic nominee is an internationalist former secretary of state whose husband, as president, began NATO's eastward expansion. Hillary Clinton undoubtedly subscribes to the 2016 Democratic platform's unequivocal pledges to stand by NATO and "deepen" alliances in Asia.

Yet her party's left wing is ascendant, with a long history of Trump-like skepticism toward U.S. alliances.

"Years after the Soviet threat has disappeared, we continue to provide European and Asian nations with military protection through our nuclear umbrella and the troops stationed in our overseas military bases," more than 50 Democratic members of Congress complained in a 2010 open letter. "Given the relative wealth of these countries, we should examine the extent of this burden that we continue to shoulder on our own dime."

Bernie Sanders was already on record, in 1997, against "wasting tens of billions of dollars helping to defend Europe." Running for commander in chief as the tribune of America's rising generations in 2016, he pointedly declined to repudiate those words.

And, of course, Clinton, under pressure from Trump and Sanders, has abandoned the Trans-Pacific Partnership trade agreement, whose significance had as much to do with cementing strategic ties with Japan as with economics.

Like those of free trade, collective security's benefits are diffuse and intangible (the absence, or limitation, of war) while its costs (dollars or, yes, lives) are concentrated and concrete. Populist backlash, accordingly, was always a latent vulnerability. Now the genie is out of the bottle.